Various sound processing and recording devices and techniques have been devised to provide a high quality, amplified output sound. A number of these devices are directed to minimizing the effects of noise which accompany the intelligible sound. In U.S. Pat. No. Re. 28,426 Dolby, signal compressor and expander circuits are described for reducing noise in a tape recorder system. The compressor circuit includes a variable frequency filter wherein the pass band is adjustable depending upon the output amplitude of the signal passed by the filter. As the amplitude of the signal passed by the filter increases, the cut-off frequency is automatically shifted more adjacent the relevant extreme of the signal band. In U.S. Pat. No. 3,789,143 to Blackmer, a compander system is provided for improving the dynamic range of a recording system. The compander is characterized by a converter which produces a signal for controlling the amplitude of an output audio signal. The control signal is a function of the RMS amplitude of an input audio signal. In U.S. Pat. No. 4,162,457 to Grodinsky, an expander circuit is disclosed for improving the dynamic qualities of the recorded signal in a two channel amplifying system. A coupling circuit interconnects the two channels. The effective coupling between the channels is dictated by the difference in amplitude of the signals from the two channel signal sources.
The present invention is directed to a sound mixing device fitted to the specialized needs of mixing audio signals produced by a relatively large number of vocalists and instrumental musicians. There are inherent difficulties in attempting to provide a high quality sound when combining an orchestra sound with that of a vocalist group. Achieving a proper balance among the various sounds produced can be an almost insurmountable task. The actual projection of brass instruments can easily mask the intelligible portion of the frequency spectrum of vocal music thereby resulting in a misunderstanding of the words sung by the vocalists. Live performances by orchestras in facilities not acoustically appropriate for orchestra sound production and the generation of nonprofessional instrumental sounds can readily cause problems in the balancing of the sound levels of the individual instruments. In addition to live performances, similar difficulties arise in connection with the recording of vocalist groups and/or instrumental musicians. This result is particularly evident when the recording of a live performance is made. A recording studio is normally designed to obviate these problems. However, the recording studio frequently does not have the capability for handling large numbers of vocalists or instrumentalists. The foregoing problems can be solved by utilizing an acoustically proper auditorium capable of housing a great number of vocalists and instrumentalists. But this solution is often unworkable since it is often difficult to provide an esthetically pleasing as well as the desired acoustical surroundings. In this light, the automatic microphone mixing apparatus combines a large number of vocalists and instrumental produced sounds without undue concern to the acoustical character of the building in which the vocalists and instrumentalists perform. Generally, the present invention is a sound reinforcement system which includes a plurality of microphones. Each microphone communicates with a volume regulating circuit having a reference level circuit. The reference level circuit outputs a reference level signal utilized in the suppressing or enhancing of audio signals. The level of each audio signal is preset to a fixed, predetermined magnitude, depending upon the magnitude of the reference level signal. Unlike previous sound mixing devices in which the level of the audio signal is variable, the present invention sets or fixes each audio signal from each of the various instrumentalists and vocalists to the same, predetermined level and then mixes these audio signals. As a result, there is no dominant instrument or vocalist sound generated.